These studies will illustrate how the arginine vasopressin (AVP) neuropeptide system responds to changes in an animal's social environment. Within a specific pathway originating in the bed nucleus of the stria terminalis (BNST) and medial amygdala (MA), AVP has been associated with aggression and parental care. The studies described in this proposal are designed to examine the flexibility of this neurotransmitter system by examining whether: (1) aggression and parental care are affected by the social environment during development, (2) flexibility in social behavior is associated with changes in AVP and (3) AVP injections produce changes in aggression and/or parental care. In order to answer these questions, this study will examine how species- typical patterns of social behavior can be altered by cross-fostering two species of mice that show different patterns of aggression and parental care. By examining the behavior of adult cross-fostered mice, this study will show whether aggression and parental care are altered by changes in the social environment during development. A second goal of this study is to use immunocytochemistry to examine whether changes in aggression and parental behavior are associated with changes in the pattern of AVP distribution within the brain. Finally, this project will examine how the context of a behavior may modulate the effects of AVP by examining the effects of intracerebroventricular AVP injections under different contexts. Together, these studies will clarify the role of AVP in regulating species-typical patterns of aggressive and parental behavior and show how developmental influences can alter a specific neurotransmitter system.